There has hitherto been known a hydraulically driven motor vehicle in which a hydraulic pump is driven by an engine and a hydraulic motor is rotated by a discharge pressure fluid from the hydraulic pump for driving a traveling body that constitutes the motor vehicle caused to travel with the hydraulic motor. In order to allow such a hydraulically driven motor vehicle to be accelerated and decelerated, the hydraulic motor has commonly been designed to be of a variable capacity type in which for the purpose of the vehicle acceleration the hydraulic motor is accelerated by increasing the capacity thereof and for the purpose of the vehicle deceleration the hydraulic motor is decelerated by reducing the capacity thereof.
As disclosed, for example, in Japanese Unexamined Patent Publication No. Sho 62-255658, it has been known that the capacity of a hydraulic pump can be gradually reduced by permitting an operating member for increasing and decreasing the capacity thereof to be operated in a direction in which the capacity is reduced and the capacity of the hydraulic pump can be swiftly increased by permitting the operating member to be operated in a direction in which the capacity is increased, thereby effecting a gradual deceleration of a motor vehicle while enhancing an ability thereof to swiftly accelerate itself.
It has also been known that a hydraulically driven motor vehicle having a hydraulic motor of variable capacity type can be provided with a servo cylinder assembly and a servo valve which are designed to control the capacity thereof in accordance with the pump pressure of a hydraulic pump and the rotation speed of an engine, in which the capacity of the hydraulic motor is reduced when the rotation speed of the engine is increased (when the vehicle is to be accelerated) to increase the rotation rate of the hydraulic motor and the capacity of the hydraulic motor is increased when the rotation rate of the engine is reduced (when the vehicle is to be decelerated) to reduce the rate of rotation of the hydraulic motor.
And, where it is desirable to change the capacity of the hydraulic motor in such a manner, it has been found that as in the case in which the capacity of the hydraulic pump is varied as described previously, not only is the ability to accelerate enhanced but also it is possible to decelerate gradually, by rapidly reducing the capacity of the hydraulic motor when the rotation speed of the engine has been increased whereas by gradually increasing the capacity of the hydraulic motor when the rotation speed of the engine has been reduced.
It has been found, however, that whereas the former system noted previously as disclosed in the Japanese Unexamined Patent Publication Sho No. 62-255658 in which the capacity of the hydraulic pump is controlled without regard to the pump pressure and the engine rotational speed makes it possible to control how to change the capacity variably as regards the directions in which it is increased and reduced, the latter system as described above in which a combination of the servo cylinder assembly and the servo valve is employed to control the capacity in accordance with the pump pressure and the engine rotational speed is not able, as it is, to control how to change the capacity variably with respect to the directions in which it is increased and decreased as in the former system.
For this reason, a measure has typically been taken in the prior art, in which a circuit for interconnecting the servo valve and the servo cylinder chambers is provided with a slow return valve whereby the speed of the servo piston at which it is displaced in a direction in which the capacity is reduced is accelerated whereas the speed of the servo piston at which it is displaced in a direction in which the capacity is increased is decelerated, thus making it possible to enhance the ability to accelerate quickly and to decelerate slowly.
It may be noted that it is preferred for the change in speed of a motor vehicle to be smaller when it is to be accelerated while it is traveling at a given speed, to be greater when it is being started from a stop, to be smaller initially when the traveling motor vehicle is being decelerated at an initial period of its deceleration, and to be greater immediately before it comes to a stop (at a final period of its deceleration).
It is also noted that in a case where a slow return valve is provided as described previously, the speed of the servo piston at which it is displaced in the direction in which the capacity is reduced will be a given value that can be preset independently of any particular position of the servo piston, as shown by the line A in FIG. 1 of the drawings attached hereto. Likewise, the speed of the servo piston at which it is displaced in the direction in which the capacity is increased will then be a constant value that can be established independently of any specific position of the servo piston, as shown by the line B in FIG. 1.
Noting in this way the fact that where a slow return valve is employed, a speed of displacement of the servo piston is rendered constant independently of its position as shown by the lines A and B in FIG. 1, it has been the prior art practice to improve as a whole both the acceleration and deceleration characteristics by adopting as the acceleration characteristic the one that can be seen intermediate between one settable in a traveling period and one settable in a starting period and as the deceleration characteristic the one that can be found intermediate between one in an initial deceleration period and one settable in a final deceleration period, as noted above. It has now been discovered that such a measure as it is cannot give rise to a fully satisfactory entire acceleration and deceleration performance for the hydraulically driven motor vehicle.
With the above described problem taken into account, it is an object of the present invention to provide an apparatus for controlling the capacity of a hydraulic motor in a hydraulically driven motor vehicle, that is capable of giving rise to a fully satisfactory entire acceleration and deceleration performance for the motor vehicle by permitting the change in speed thereof to be smaller while it is traveling, to be greater while it is being accelerated in a period in which it is started, to be smaller in an initial period of its deceleration, and to be increased in a final period of the deceleration.